Earth science

Gilla:		Dela:

Video

Materials

• 1 0.5 L (16.9 fl oz) plastic bottle
• 1 matchbox
• Water
• Safety equipment: 1 fire extinguisher

Short explanation

When you reduce the pressure in the bottle, it also cools down, which leads to water vapor in the bottle condensing into small water drops that are visible as a cloud.

Long explanation

When you pour water into the bottle, some of the water will change from a liquid state to a gaseous state (evaporates) and become invisible water vapor in the bottle. A liquid always changes to a gaseous state to some extent, and vice versa, even if no change in temperature occurs. However, this so-called chemical equilibrium shifts in some direction when a temperature change occurs. There are now two things in the bottle's "atmosphere"; the gas mixture we know as air, plus water vapor. Water vapor, i.e. water in gaseous form, is a natural part of the atmosphere and is often considered a part of it.

When you squeeze the bottle, the temperature inside increases. This is because the air molecules inside get closer to each other and collide more and more violently, which leads to their speed increasing - and temperature is a measure of how fast particles move. The air temperature may rise by maybe 0.5-1 degree. This leads to the water in the bottle also heating up and more liquid water evaporating to water vapor. When you then relieve the pressure, the temperature drops again, as the air molecules now come further apart. This causes a large amount of water vapor to condense into liquid water, and small water droplets that are visible as a cloud are formed.

The role of the matches in this demonstration is to make small "pollutants" in the air, which increases the condensation of water. This is because water more easily condenses on small particles, such as dust or pollen. This is because a small water droplet has a large surface area compared to its volume, which leads to it evaporating quickly. But if the droplet is formed around a pollutant it is larger from the start - has a smaller surface area compared to its volume - and survives more easily.

The cloud in the bottle has formed in the same way as clouds in the atmosphere. In nature, water constantly evaporates from Earth's surface by the Sun warming it. This water ends up as invisible water vapor in the air just above the ground. This air is also heated by Earth's surface, which causes it to rise and take the water vapor higher into the atmosphere. Up there it's cold and the water vapor turns into liquid water again (condenses). Now small water droplets are formed high up in the atmosphere, which form the clouds (some water droplets also freeze into small ice crystals).

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you change the amount of water?
• What happens if you change the number of matches?
• What happens if you change the temperature of the water?
• What happens if you change the size of the bottle?
• What happens if you use food coloring in the water?

Variation

You can also make a cloud in a bottle using ethanol and a pump. You can find this experiment here: Cloud in a bottle 2.